Laminate films are widely used in a variety of packaging applications. Good mechanical properties such as elongation, tensile strength, dart impact strength, and puncture resistance are desired to ensure package integrity, especially during packaging and transportation. In flexible laminate film structures, a sealant film is adhered to a substrate film commonly made of biaxially oriented polyester, biaxially oriented polypropylene, or biaxially oriented polyamide. Multilayer structures have been developed for use in sealant films which has tremendously improved the mechanical properties of sealant films relative to those made with the same composition in a monolayer construction.
Ethylene polymers, such as low density polyethylene (LDPE), linear low density polyethylene (LLDPE) prepared by Ziegler-Natta catalyst in a gas phase process, and blends thereof are generally employed in the art to form a sealant film. While such ethylene polymers work reasonably well because they provide relatively low-cost solutions, their properties render them less preferred than other polyethylenes for a number of applications. Efforts to address disadvantages caused by LDPE and LLDPE include incorporating metallocene polyethylenes (mPEs) in sealant films. However, whatever progress a sealant film has gained in terms of mechanical properties by the above solutions, once a sealant film is laminated to a substrate film, the resulting characteristics of a laminate film, depending upon the specific substrate film used, reflect very limited improvements. Therefore, it is difficult for laminate film manufacturers to achieve significant improvements in mechanical properties by exploring alternatives in sealant films.
WO 2014/042898 provides ethylene-based copolymers, particularly ethylene-based polymers having about 80.0 to 99.0 wt % of polymer units derived from ethylene and about 1.0 to about 20.0 wt % of polymer units derived from one or more C3 to C20 α-olefin comonomers; the ethylene-based polymer having a local maximum loss angle at a complex modulus, G*, of 2.50×104 to 1.00×106 Pa and a local minimum loss angle at a complex modulus, G*, of 1.00×104 to 3.00*×104 Pa. This patent application also includes articles, such as films, produced from such polymers and methods of making such articles.
U.S. Patent Publication No. 2013/211008 discloses polyethylene compositions comprising one or more ethylene polymers and one or more dendritic hydrocarbon polymer modifiers, in particular, this patent application further relates to polyethylene blends comprising one or more ethylene polymers and one or more dendritic hydrocarbon polymer modifiers, wherein the modifier has: 1) a g′ value less than 0.75; 2) a Cayley tree topology with a layer number of 2 or more; and 3) an average Mw between the branch points of 1,500 g/mol or more.
U.S. Patent Publication No. 2012/0100356 relates to a multilayer blown film with improved strength or toughness comprising a layer comprising a metallocene polyethylene (mPE) having a high melt index ratio (MIR), a layer comprising an mPE having a low MIR, and a layer comprising a HDPE, and/or LDPE. Other embodiments have skin layers and a plurality of sublayers. At least one sublayer includes an mPE, and at least one additional sublayer includes HDPE and/or LDPE. The mPE has a density from about 0.910 to about 0.945 g/cm3, MI from about 0.1 to about 15, and melt index ratio (MIR) from about 15 to 25 (low-MIR mPE) and/or from greater than 25 to about 80 (high-MR mPE). The process is related to supplying respective melt streams for coextrusion at a multilayer die to form a blown film having the inner and outer skin layers and a plurality of sublayers, wherein the skin layers and at least one of the sublayers comprise mPE and at least one of the sublayers comprise HDPE, LDPE or both. Draw-down, blow-up ratios and freeze-line distance from the die are controlled to facilitate a high production rate.
U.S. Pat. No. 8,586,676 provides a polymer composition and articles made therefrom. The composition includes: (a) a polyethylene having (i) at least 50 wt % ethylene moieties; and (ii) up to 50 wt % of a C3 to C20 comonomer moieties, a density of about 0.860 to about 0.965 g/cm3, a melt index of about 0.1 to about 10.0 g/10 min and a branching index of about 0.96 to about 1.0; and (b) a polyethylene having: (i) at least 65 wt % ethylene moieties; and (ii) up to 35 wt % of a C3 to C20 comonomer moieties, the wt % s based upon the total weight of the latter polyethylene, a density of about 0.905 to about 0.945 g/cm3, a melt index (MI) of about 0.1 to about 10.0 g/10 min, and a branching index (g′) of about 0.7 to about 0.95.
Numerous attempts have been made to improve mechanical properties of laminated films by modifying compositions of the sealant films to be laminated to substrate films, yet a need for a sealant film to better overcome the counteracting effect that the substrate film places on the formed laminate film remains. Applicant has found that such objective can be achieved by applying a polyethylene comprising a copolymer derived from ethylene and one or more C3 to C20 α-olefin comonomers in each of the two outer layers and the core layer, particularly in a blend with another polyethylene at a certain ratio, to prepare a multilayer film. When laminated to a substrate film, the multilayer film made of the above composition, even at a thinner gauge, can show a remarkable advantage in low-temperature bag drop performance over a conventional multilayer film. This makes the inventive multilayer film well suited for specific packaging applications such as a freezer film. As an indicator for the combined effect of multiple mechanical properties, including tensile strength at break, dart impact strength, puncture resistance and tear resistance, this enhanced bag drop performance may reverse the perception about the limit of achievable mechanical properties of laminate films. Therefore, by adjusting compositions in different layers with the currently available selection of ethylene polymers, a laminate structure with the inventive film can be produced to provide desired mechanical properties independently of the chosen substrate film.